Tiafoe, the No.11 seed, beat Michelsen 5–7, 3–6, 7–5, 6–3, 7–6(6). He had fallen behind 3–0 in the third set before turning the match around. The victory put him in a US Open semifinal for the third time.
Shelton’s route was just as demanding. He beat Alcaraz 6–7(5), 6–1, 6–3, 1–6, 7–6(7), closing the match with an ace. The finish was the latest in US Open history, leaving recovery as part of the preparation for Friday’s contest.
The previous results establish how little room either player has had to relax. They also leave a clear distinction between reaching this stage and winning it: neither quarterfinal escape gives its author an advantage on Friday’s scoreboard.
The semifinal follows the afternoon meeting between Alexander Zverev and Karen Khachanov. By the end of the night, one of the two Americans will have earned the chance to play for the championship. Their recent meetings in New York suggest that earning it may require another long evening.